Base station antennas for cellular communications systems typically include one or more linear arrays of radiating elements such as dipoles that are mounted on, for example, a flat panel. Each array of radiating elements may produce an antenna beam that has desired characteristics such as, for example, a desired beam elevation angle, beam azimuth angle, and/or half power beam width in order to provide cellular service to a specified coverage area. A signal that is to be transmitted by one of the linear arrays of such a base station antenna is divided into multiple sub-components, and each sub-component may be fed through an antenna feed network to a respective one of the radiating elements.
Based on network coverage requirements, cellular operators may find it advantageous to adjust the vertical elevation angle (i.e., the vertical angle of the antenna with respect to the horizon) or “tilt” of the main beam of a linear array in order to change the coverage area of the antenna. Such adjustment is typically referred to as “down-tilting” as the antenna beam is typically tilted to point at an elevation angle of 0° or less with respect to the horizon such as, for example, an elevation angle of 0° to −10°. The base station antenna may be electronically down-tilted by controlling the phases of the sub-components of the signal that are transmitted through the respective radiating elements of the linear array that forms the antenna beam in a manner that changes the elevation angle of the main antenna beam. Such electronic down-tilt is typically performed by transmitting a control signal from a remote location to the base station antenna. In response to this control signal, the base station antenna adjusts settings of adjustable phase shifters that are included in the antenna feed network to implement the phase shifts that down-tilt the main beam of the linear array at issue.
Electromechanical phase shifters are typically used to implement the adjustable phase shifters that are used to electronically down-tilt the antenna beams of the linear antennas. An example of such an electromechanical phase shifter is the wiper arc phase shifter disclosed in U.S. Pat. No. 7,463,190. The phase shifter of the '190 patent has a stationary “main” printed circuit board and a mechanically rotatable “wiper” printed circuit board mounted thereon. The main printed circuit board has an input, a relatively large number (e.g., five, seven or nine) of outputs, and a plurality of arced transmission paths that connect to the respective outputs. The arced transmission paths are arranged as concentric arcs having different radii, and hence each arced transmission path has a different length. An RF signal that is input at the input of the phase shifter is split into sub-components and at least some of these sub-components are transferred to the wiper printed circuit board, where they capacitively couple onto the respective arced transmission paths on the main printed circuit board. In this fashion, the outputs of the phase shifter may be coupled to the input by respective RF transmission paths that have different lengths. Since the length of a transmission path effects the phase of an RF signal transmitted therethrough, the different length RF transmission paths may apply a linear phase taper to the sub-components of the input RF signal. Moreover, the amount of phase shift applied to each sub-component of the input RF signal may be adjusted by mechanically moving the wiper printed circuit board to change the position along the arced transmission paths where the wiper printed circuit board capacitively couples to the main printed circuit board. Each of the outputs of the phase shifter may be connected to a respective one of the radiating elements or to a respective sub-groups of radiating elements of the linear array so that a linear phase taper may be applied to the radiating elements (or sub-groups thereof).
Base station antennas that use electromechanical phase shifters typically include a plurality of Remote Electronic Tilt (RET) units that are used to move the wiper printed circuit boards of the phase shifters. Each RET unit may include one or more motors such as direct current (DC) motors or stepper motors. In some cases a motor may be shared over multiple RET units. Mechanical linkages connect each motor to a respective one of the phase shifters (or to two of the phase shifters when dual polarized radiating elements are used, as the same phase shift is typically applied to the signals of each polarization) so that the motors may be used to move the wiper boards of the phase shifters. The electrical down-tilt is effected by sending a control signal to the base station antenna. This control signal is transmitted over an Antenna Interface Standards Group (AISG) control channel to a RET controller included in the base station antenna. The RET controller includes software that decodes and processes AISG commands that are included in the AISG control signal and, in response thereto, transmits control signals to the individual RET units. The control signals transmitted by the RET controller to an individual RET unit may activate a motor of the RET unit to drive a mechanical linkage to adjust an associated electromechanical phase shifter to apply a desired phase taper to the radio frequency (RF) signals input thereto. Thus, a RET unit is a device that is used to physically adjust a phase shifter of the base station antenna while the RET controller is a unit that receives AISG commands and controls one or more RET units in response thereto.